The present invention relates to a die casting method and apparatus for manufacturing high quality castings having excellent mechanical properties.
As is well known, a die casting method is a casting method in which molten metal within a casting sleeve is pressure-charged into the cavity of a die and is solidified to thereby manufacture a casting.
The die casting method has advantages that obtained castings have high dimensional accuracy, mass production is possible because the method allows high speed operation, and fully automatic operation is possible through use of a computer. Therefore, the die casting method is frequently used for casting of low-melting-point metals such as aluminum alloys.
However, there have been pointed out the following problems in relation to the die casting method.
A first problem relates to strength. That is, unless a casting obtained through use of the die casting is subjected to reforming such as heat treatment, the casting is generally inapplicable to high-strength members that must have high strength. The reason for this is as follows.
In general, when die casting is performed, molten metal poured into the casting sleeve is rapidly cooled by means of the inner wall of the casting sleeve, and thus solidification scale is generated. Since the solidification scale is cast together with said molten metal, the resultant product contains the solidification scale, resulting in a decrease in the mechanical strength of the product.
Further, when molten metal is injected from the sleeve into a die, air within the casting sleeve becomes caught in said molten metal and is mixed into a resultant casting. In this case, when the casting is heat-treated, swelling called a blister is generated, which becomes a cause of deterioration of quality.
In order to solve the above-described problems of the die casting method, various types of special die casting methods have been proposed. Among them, a hot sleeve method is a die casting method in which casting is performed while a casting sleeve is heated in order to prevent generation of solidification scale at the inner wall of the casting sleeve.
Also, a vertical-injection die casting method is performed in order to suppress catching of air within the casting sleeve.
However, the above-described various types of special die casting methods have the following problems to be solved.
That is, when the speed of injection from a casting sleeve into a die cavity is increased in order to enhance productivity, molten metal within the casing sleeve undergoes turbulent flow, so that the amount of air caught in said molten metal increases, and in addition solidification scale that is produced by rapid cooling and solidification of molten metal at the inner surface of the die is taken into a product. This causes deterioration of the mechanical properties of the obtained product.
Meanwhile, when molten metal is injected from the casting sleeve into the die cavity at a slow speed in order to prevent catching of air, run of molten metal within the die cavity becomes poor, which becomes a cause of a product defect such as misrun.
Japanese Patent Application Laid-Open No. 8-257722 discloses a die casting method that attempts to solve the above-described problems involved in the various kinds of conventional special die casting methods.
In the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, primary crystals of molten metal are granulated within a casting sleeve, charged under pressure into the cavity of a die in a semi-molten state, and solidified therein. According to the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, die casting is performed in the steps described below.
First, as shown in FIG. 8, molten metal maintained at a temperature near the liquidus line is poured into a casting sleeve 2. Subsequently, as shown in FIG. 8, the temperature of said molten metal within the casting sleeve 2 is decreased at a predetermined cooling rate, from the temperature near the liquidus line to a predetermined temperature that is below the liquidus line but higher than the solidus line or eutectic line, in order to substantially granulate primary crystals of said molten metal, thereby bringing said molten metal into a semi-molten state. With this operation, there can be obtained thixotropic fluid composed of granular primary crystals and liquid having a temperature not less than the eutectic temperature.
Subsequently, as shown in FIG. 8, the semi-molten metal is charged from the casting sleeve 2 into a die 1. At this time, the semi-molten metal charged from the casting sleeve 2 into the die 1 undergoes laminar flow due to its thixotropy, so that the amount of gas caught in the semi-molten metal decreases. That is, when the metallographic structure is granulated with resultant formation of a solid phase, even if some force would be added, movement of the granulated solid phase and movement of the liquid phase occur simultaneously, so that there occurs a phenomenon in which the solid and liquid phases move together. As a result, catching of gas occurs to a lesser extent, and therefore the amount of gas contained in a casting decreases with the result that blisters are not generated even when heat treatment is performed.
However, the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722 has the following drawbacks that must be overcome.
In the die casting method disclosed in Japanese Patent Application Laid-Open No. 8-257722, as shown in FIG. 8, molten metal is poured into the casting sleeve 2 from above through use of a ladle or the like. Therefore, when said molten metal falls into the interior of the sleeve 2, it undergoes turbulent flow within the sleeve 2 and air may be caught in said molten metal. In this case, the amount of gas contained in said molten metal increases and oxide film tends to be formed on the surface of said molten metal, so that gas holes are produced. When strict quality control is performed in order to prevent generation of such gas holes, yield decreases. Further, since casting must be controlled in order to prevent oxides produced in said molten metal from being caught in said molten metal, which oxides would otherwise affect the mechanical properties, the production cycle time may increase, and yield may decrease due to strict quality control.
FIG. 9 shows an example of oxide film 30 and a gas hole 31 which decrease the yield of products as a result of performance of strict quality control.
The die casting method of the present invention was accomplished in view of the forgoing problems of prior art techniques, and an object of the present invention is to provide a die casting method which can minimize the amount of air caught in molten metal when fed into a casting sleeve in order to reduce the amount of gas contained in said molten metal to thereby prevent generation of oxide film or gas holes, while solving problems such as air catching occurring at the time of injection into the cavity of the die and molten metal run defect, thereby enabling efficient production of defect-free perfect castings and increasing the yield. Another object of the present invention is to provide die castings obtained through use of the die casting method.
To solve the above-described problems, the present invention provides in a die casting method that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals,
the die casting method characterized in that said molten metal is fed into a casting sleeve through its side portion in the vicinity of the bottom portion thereof, and an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe.
In the die casting method of the present invention, primary crystals of molten metal within the casting sleeve are substantially granulated and thus said molten metal is brought into a semi-molten state. Subsequently, said molten metal is charged under pressure into the cavity of a die and solidified. The supply of the inert gas near the molten-metal feed port into the molten-metal feed pipe is performed with the feed of said molten metal into the casting sleeve through its side portion in the vicinity of the bottom portion thereof. Therefore, oxidation of said molten metal in the semi-molten near the molten-metal feed port state occurs to a lesser extent. The feed of said molten metal into the casting sleeve is performed through a side portion of the sleeve near the bottom portion thereof. Therefore, oxidation of said molten metal in the semi-molten state occurs to a lesser extent, so that stable mechanical properties are attained.
Also, the present invention provides in a die casting method that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals, the die casting method characterized in that said molten metal is fed into a casting sleeve through a portion that is offset from the center position between the rest position of a plunger tip disposed within the sleeve and a die toward the plunger tip; and an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe.
In the die casting method of the present invention, primary crystals of molten metal within the casting sleeve are substantially granulated and thus said molten metal is brought into a semi-molten state. Subsequently, said molten metal is charged under pressure into the cavity of a die and solidified. The supply of the inert gas near the molten-metal feed port into the molten-metal feed pipe is performed with the feed of said molten metal into a casting sleeve through a portion that is offset from the center position between the rest position of the plunger tip disposed within the sleeve and the die toward the plunger tip. Therefore, oxidation of said molten metal in the semi-molten near the molten-metal feed port state occurs to a lesser extent. The feed of said molten metal into the casting sleeve is performed through a portion that is offset from the center position between the rest position of the plunger tip and the die toward the plunger tip. Therefore, oxidation of said molten metal in the semi-solidified state occurs to a lesser extent, so that stable mechanical properties are attained.
Further, the present invention provides in a die casting method that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals, the die casting method characterized in that said molten metal is fed into a casting sleeve through its side portion in the vicinity of the bottom portion thereof while undergoing laminar flow; and an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe.
In the die casting method of the present invention, primary crystals of molten metal within the casting sleeve are substantially granulated and thus said molten metal is brought into a semi-molten state. Subsequently said molten metal is charged under pressure into the cavity of a die and solidified. The supply of the inert gas near the molten-metal feed port into the molten-metal feed pipe is performed with the feed of said molten metal into the casting sleeve through its side portion in the vicinity of the bottom portion thereof while undergoing laminar flow. Therefore, oxidation of said molten metal in the semi-molten near the molten-metal feed port state occurs to a lesser extent. The feed of said molten metal into the casting sleeve is performed in a laminar flow state through a side portion of the sleeve near the bottom portion thereof. Therefore, oxidation of said molten metal in the semi-molten state occurs to a lesser extent, so that stable mechanical properties are attained. Especially, since casting is performed while said molten metal undergoes laminar flow, the amount of air caught in molten metal can be reduced compared to the case where casting is performed while said molten metal undergoes turbulent flow. Thus, the amount of oxides and the like contained in castings can be decreased.
Further, the die casting method according to the present invention is characterized in that the rate of cooling molten metal within the sleeve is controlled to be less than 10xc2x0 C./sec.
When the rate of cooling molten metal within the sleeve is made less than 10xc2x0 C./sec, produced primary crystals can be granulated. Further, the rate of cooling molten metal within the sleeve is preferably set to be greater than 1.7xc2x0 C./sec. In this case, productivity can be improved within a range in which produced primary crystals can be granulated.
Specific methods for performing cooling at a cooling rate within a predetermined range are as follows:
(1) The sleeve is formed of a material of low heat conductivity such as ceramics in order to decrease the cooling rate at the surface of the sleeve, thereby making the inside cooling rate less than 10xc2x0 C./sec. When the inside cooling rate becomes less than 1.7xc2x0 C./sec, the sleeve cooling system is needed.
(2) When a metallic sleeve is used, the metallic sleeve is heated in advance in order to increase the initial temperature. Especially, in the case of A357 material (having the composition (by weight %) of 6.5-7.5% Si, 0.60% Mg, 0.12% Fe, 0.10% Cu, 0.05% Mn and balance substantially Al), the initial temperature of the sleeve is held at not less than 200xc2x0 C. When the cooling rate inside said molten metal becomes less than 1.7-10xc2x0 C./sec, the sleeve is cooled.
(3) A cooling container is formed into a cold crucible structure, and the surface of molten metal is heated through high frequency agitation, so that heat is applied to said molten metal while the container is cooled. Thus, the cooling rate at the surface of said molten metal is controlled, and the inside portion of said molten metal is cooled at a predetermined cooling rate.
In the present invention, the semi-molten metal granulated within the casting sleeve is preferably formed into a spherical shape when the semi-molten metal is charged into the cavity of a die. In this case, since the granules become finer, run of said molten metal is improved.
Further, the die casting method according to the present invention is characterized in that the die casting is performed under control such that the total amount of gas contained in an obtained casting does not exceed about 1 cc/100 g.
As a result of control such that the total amount of gas contained in an obtained casting does not exceed about 1 cc/100 g, there can be obtained a casting whose total amount of gas contained therein is reduced. Further, when the die casting method of the present invention is employed, control of the total amount of gas can be performed quite efficiently.
Further, in the die casting method according to the present invention, the interior of the casting sleeve is made an inert gas atmosphere at least when molten metal is fed into the sleeve. Therefore, generation of gas defects can be prevented. In addition, oxidation of said molten metal can be minimized.
Further, the present invention provides in a die casting obtained such a manner that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals, the die casting characterized by being obtained such a manner that said molten metal is fed into a casting sleeve through its side portion in the vicinity of the bottom portion thereof, an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe; and that control is performed such that the total amount of gas contained in the casting does not exceed about 1 cc/100 g.
The die casting of the present invention that is produced in accordance with the die casting method of the present invention under control such that the total amount of gas contained in the casting does not exceed about 1 cc/100 g is low in cost because there is employed means for supplying the inert gas near the molten-metal feed port into the molten-metal feed pipe and feeding said molten metal into the casting sleeve through its side portion in the vicinity of the bottom portion thereof, and therefore an unduly complicated casting process is not required. Therefore, the die casting has stable mechanical properties because of its reduced total amount of gas.
Further, the present invention provides in a die casting obtained such a manner that after molten metal is fed into a casting sleeve through its side portion, said molten metal is cooled in order to granulate crystallized primary crystals, the die casting characterized by being obtained such a manner that said molten metal is fed into a casting sleeve through a portion that is offset from the center position between the rest position of a plunger tip disposed within the sleeve and a die toward the plunger tip, an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe; and that control is performed such that the total amount of gas contained in the casting does not exceed about 1 cc/100 g.
The die casting of the present invention that is produced in accordance with the die casting method of the present invention under control such that the total amount of gas contained in the casting does not exceed about 1 cc/100 g is low in cost because there are employed means for supplying the inert gas near the molten-metal feed port into the molten-metal feed pipe and feeding said molten metal into the casting sleeve through a portion that is offset from the center position between the rest position of a plunger tip disposed within the sleeve and the die toward the plunger tip and employed means for feeding molten metal into the casting sleeve through a portion that is offset from the center position between the rest position of the plunger tip and the die toward the plunger tip, and therefore an unduly complicated casting process is not required. In addition, the semi-solidified molten metal undergoes oxidation to a lesser extent, and therefore the die casting has stable mechanical properties.
Further, the present invention provides in a die casting obtained such a manner that after molten metal is fed into a casting sleeve through its side portion, the molten metal is cooled in order to granulate crystallized primary crystals,
the die casting characterized by being obtained such a manner that said molten metal is fed into a casting sleeve in a laminar flow state through a side portion in the vicinity of the bottom portion thereof, an inert gas is supplied near a molten-metal feed port into a molten-metal feed pipe; and that control is performed such that the total amount of gas contained in the casting does not exceed about 1 cc/100 g.
The die casting of the present invention that is produced in accordance with the die casting method of the present invention under control such that the total amount of gas contained in an obtained casting does not exceed about 1 cc/100 g is low in cost because there are employed means for supplying the inert gas near the molten-metal feed port into the molten-metal feed pipe and feeding said molten metal into the casting sleeve in a laminar flow state through a side portion in the vicinity of the bottom portion thereof and employed means for feeding molten metal into the casting sleeve in a laminar flow state through a side portion in the vicinity of the bottom portion thereof, and therefore an unduly complicated casting process is not required. In addition, the semi-solidified molten metal undergoes oxidation to a lesser extent, and therefore the die casting has stable mechanical properties. Moreover, since casting is performed while said molten metal undergoes laminar flow, the amount of air caught in molten metal can be reduced. Thus, the amount of oxides contained in the casting can be decreased.